graphql-engine/server/src-lib/Hasura/Backends/Postgres/Instances/Schema.hs

{-# OPTIONS_GHC -fno-warn-orphans #-}
{-# LANGUAGE UndecidableInstances #-}

module Hasura.Backends.Postgres.Instances.Schema
  (
  ) where

import           Hasura.Prelude

import qualified Data.Aeson                             as J
import qualified Data.HashMap.Strict                    as Map
import qualified Data.HashMap.Strict.Extended           as M
import qualified Data.HashMap.Strict.InsOrd.Extended    as OMap
import qualified Data.List.NonEmpty                     as NE
import qualified Data.Text                              as T
import qualified Language.GraphQL.Draft.Syntax          as G

import           Data.Has
import           Data.Parser.JSONPath
import           Data.Text.Extended
import           Data.Typeable

import qualified Hasura.GraphQL.Parser                  as P
import qualified Hasura.GraphQL.Schema.Backend          as BS
import qualified Hasura.GraphQL.Schema.Build            as GSB
import qualified Hasura.RQL.IR.Select                   as IR
import qualified Hasura.RQL.IR.Update                   as IR
import qualified Hasura.SQL.AnyBackend                  as AB

import           Hasura.Backends.Postgres.SQL.DML       as PG hiding (CountType)
import           Hasura.Backends.Postgres.SQL.Types     as PG hiding (FunctionName, TableName)
import           Hasura.Backends.Postgres.SQL.Value     as PG
import           Hasura.Backends.Postgres.Types.BoolExp
import           Hasura.Backends.Postgres.Types.Column
import           Hasura.Base.Error
import           Hasura.GraphQL.Parser                  hiding (EnumValueInfo, field)
import           Hasura.GraphQL.Parser.Internal.Parser  hiding (field)
import           Hasura.GraphQL.Schema.Backend          (BackendSchema, ComparisonExp,
                                                         MonadBuildSchema)
import           Hasura.GraphQL.Schema.BoolExp
import           Hasura.GraphQL.Schema.Common
import           Hasura.GraphQL.Schema.Select
import           Hasura.GraphQL.Schema.Table
import           Hasura.RQL.IR
import           Hasura.RQL.Types
import           Hasura.SQL.Tag
import           Hasura.SQL.Types


----------------------------------------------------------------
-- BackendSchema instance

-- | This class is an implementation detail of 'BackendSchema'.
-- Some functions of 'BackendSchema' differ across different Postgres "kinds",
-- or call to functions (such as those related to Relay) that have not been
-- generalized to all kinds of Postgres and still explicitly work on Vanilla
-- Postgres. This class alllows each "kind" to specify its own specific
-- implementation. All common code is directly part of `BackendSchema`.
class PostgresSchema (pgKind :: PostgresKind) where
  pgkBuildTableRelayQueryFields
    :: BS.MonadBuildSchema ('Postgres pgKind) r m n
    => SourceName
    -> SourceConfig ('Postgres pgKind)
    -> TableName ('Postgres pgKind)
    -> TableInfo ('Postgres pgKind)
    -> G.Name
    -> NESeq (ColumnInfo ('Postgres pgKind))
    -> SelPermInfo ('Postgres pgKind)
    -> m [FieldParser n (QueryRootField UnpreparedValue)]
  pgkBuildFunctionRelayQueryFields
    :: BS.MonadBuildSchema ('Postgres pgKind) r m n
    => SourceName
    -> SourceConfig ('Postgres pgKind)
    -> FunctionName ('Postgres pgKind)
    -> FunctionInfo ('Postgres pgKind)
    -> TableName ('Postgres pgKind)
    -> NESeq (ColumnInfo ('Postgres pgKind))
    -> SelPermInfo ('Postgres pgKind)
    -> m [FieldParser n (QueryRootField UnpreparedValue)]
  pgkRelayExtension
    :: Maybe (XRelay ('Postgres pgKind))
  pgkNode
    :: BS.MonadBuildSchema ('Postgres pgKind) r m n
    => m (Parser 'Output n
          (HashMap
            ( TableName ('Postgres pgKind)
            )
            ( SourceName
            , SourceConfig ('Postgres pgKind)
            , SelPermInfo  ('Postgres pgKind)
            , PrimaryKeyColumns ('Postgres pgKind)
            , AnnotatedFields ('Postgres pgKind)
            )
          )
         )

instance PostgresSchema 'Vanilla where
  pgkBuildTableRelayQueryFields    = buildTableRelayQueryFields
  pgkBuildFunctionRelayQueryFields = buildFunctionRelayQueryFields
  pgkRelayExtension = Just ()
  pgkNode = nodePG

instance PostgresSchema 'Citus where
  pgkBuildTableRelayQueryFields     _ _ _ _ _ _ _ = pure []
  pgkBuildFunctionRelayQueryFields  _ _ _ _ _ _ _ = pure []
  pgkRelayExtension = Nothing
  pgkNode = undefined


-- postgres schema

instance
  ( HasTag   ('Postgres pgKind)
  , Typeable ('Postgres pgKind)
  , Backend  ('Postgres pgKind)
  , PostgresSchema pgKind
  ) => BackendSchema ('Postgres pgKind) where
  -- top level parsers
  buildTableQueryFields          = GSB.buildTableQueryFields
  buildTableRelayQueryFields     = pgkBuildTableRelayQueryFields
  buildTableInsertMutationFields = GSB.buildTableInsertMutationFields
  buildTableUpdateMutationFields = GSB.buildTableUpdateMutationFields
  buildTableDeleteMutationFields = GSB.buildTableDeleteMutationFields
  buildFunctionQueryFields       = GSB.buildFunctionQueryFields
  buildFunctionRelayQueryFields  = pgkBuildFunctionRelayQueryFields
  buildFunctionMutationFields    = GSB.buildFunctionMutationFields

  -- table components
  tableArguments = defaultTableArgs

  -- backend extensions
  relayExtension         = pgkRelayExtension @pgKind
  nodesAggExtension      = Just ()
  nestedInsertsExtension = Just ()

  -- indivdual components
  columnParser              = columnParser
  jsonPathArg               = jsonPathArg
  orderByOperators          = orderByOperators
  comparisonExps            = comparisonExps
  updateOperators           = updateOperators
  mkCountType               = mkCountType
  aggregateOrderByCountType = PG.PGInteger
  computedField             = computedFieldPG
  node                      = pgkNode

  -- SQL literals
  columnDefaultValue = const PG.columnDefaultValue


----------------------------------------------------------------
-- Top level parsers

buildTableRelayQueryFields
  :: forall pgKind m n r
   . MonadBuildSchema ('Postgres pgKind) r m n
  => SourceName
  -> SourceConfig ('Postgres pgKind)
  -> TableName    ('Postgres pgKind)
  -> TableInfo    ('Postgres pgKind)
  -> G.Name
  -> NESeq (ColumnInfo ('Postgres pgKind))
  -> SelPermInfo  ('Postgres pgKind)
  -> m [FieldParser n (QueryRootField UnpreparedValue)]
buildTableRelayQueryFields sourceName sourceInfo tableName tableInfo gqlName pkeyColumns selPerms = do
  let
    mkRF = RFDB sourceName
             . AB.mkAnyBackend
             . SourceConfigWith sourceInfo
             . QDBR
    fieldName = gqlName <> $$(G.litName "_connection")
    fieldDesc = Just $ G.Description $ "fetch data from the table: " <>> tableName
  fmap afold
    $ optionalFieldParser (mkRF . QDBConnection)
    $ selectTableConnection sourceName tableInfo fieldName fieldDesc pkeyColumns selPerms

buildFunctionRelayQueryFields
  :: forall pgKind m n r
   . MonadBuildSchema ('Postgres pgKind) r m n
  => SourceName
  -> SourceConfig ('Postgres pgKind)
  -> FunctionName ('Postgres pgKind)
  -> FunctionInfo ('Postgres pgKind)
  -> TableName    ('Postgres pgKind)
  -> NESeq (ColumnInfo ('Postgres pgKind))
  -> SelPermInfo  ('Postgres pgKind)
  -> m [FieldParser n (QueryRootField UnpreparedValue)]
buildFunctionRelayQueryFields sourceName sourceInfo functionName functionInfo tableName pkeyColumns selPerms = do
  funcName <- functionGraphQLName @('Postgres pgKind) functionName `onLeft` throwError
  let
    mkRF = RFDB sourceName
             . AB.mkAnyBackend
             . SourceConfigWith sourceInfo
             . QDBR
    fieldName = funcName <> $$(G.litName "_connection")
    fieldDesc = Just $ G.Description $ "execute function " <> functionName <<> " which returns " <>> tableName
  fmap afold
    $ optionalFieldParser (mkRF . QDBConnection)
    $ selectFunctionConnection sourceName functionInfo fieldName fieldDesc pkeyColumns selPerms


----------------------------------------------------------------
-- Individual components

columnParser
  :: (MonadSchema n m, MonadError QErr m)
  => ColumnType ('Postgres pgKind)
  -> G.Nullability
  -> m (Parser 'Both n (ValueWithOrigin (ColumnValue ('Postgres pgKind))))
columnParser columnType (G.Nullability isNullable) =
  -- TODO(PDV): It might be worth memoizing this function even though it isn’t
  -- recursive simply for performance reasons, since it’s likely to be hammered
  -- during schema generation. Need to profile to see whether or not it’s a win.
  peelWithOrigin . fmap (ColumnValue columnType) <$> case columnType of
    ColumnScalar scalarType -> possiblyNullable scalarType <$> do
      -- We convert the value to JSON and use the FromJSON instance. This avoids
      -- having two separate ways of parsing a value in the codebase, which
      -- could lead to inconsistencies.
      --
      -- The mapping from postgres type to GraphQL scalar name is done by
      -- 'mkScalarTypeName'. This is confusing, and we might want to fix it
      -- later, as we will parse values differently here than how they'd be
      -- parsed in other places using the same scalar name; for instance, we
      -- will accept strings for postgres columns of type "Integer", despite the
      -- fact that they will be represented as GraphQL ints, which otherwise do
      -- not accept strings.
      --
      -- TODO: introduce new dedicated scalars for Postgres column types.
      name <- mkScalarTypeName scalarType
      let schemaType = P.NonNullable $ P.TNamed $ P.mkDefinition name Nothing P.TIScalar
      pure $ Parser
        { pType = schemaType
        , pParser = valueToJSON (P.toGraphQLType schemaType) >=> \case
            J.Null -> parseError $ "unexpected null value for type " <>> name
            value  -> runAesonParser (parsePGValue scalarType) value
                      `onLeft` (parseErrorWith ParseFailed . qeError)
        }
    ColumnEnumReference (EnumReference tableName enumValues) ->
      case nonEmpty (Map.toList enumValues) of
        Just enumValuesList -> do
          name <- qualifiedObjectToName tableName <&> (<> $$(G.litName "_enum"))
          pure $ possiblyNullable PGText $ P.enum name Nothing (mkEnumValue <$> enumValuesList)
        Nothing -> throw400 ValidationFailed "empty enum values"
  where
    possiblyNullable scalarType
      | isNullable = fmap (fromMaybe $ PGNull scalarType) . P.nullable
      | otherwise  = id
    mkEnumValue :: (EnumValue, EnumValueInfo) -> (P.Definition P.EnumValueInfo, PGScalarValue)
    mkEnumValue (EnumValue value, EnumValueInfo description) =
      ( P.mkDefinition value (G.Description <$> description) P.EnumValueInfo
      , PGValText $ G.unName value
      )

jsonPathArg
  :: MonadParse n
  => ColumnType ('Postgres pgKind)
  -> InputFieldsParser n (Maybe (IR.ColumnOp ('Postgres pgKind)))
jsonPathArg columnType
  | isScalarColumnWhere PG.isJSONType columnType =
      P.fieldOptional fieldName description P.string `P.bindFields` fmap join . traverse toColExp
  | otherwise = pure Nothing
  where
    fieldName = $$(G.litName "path")
    description = Just "JSON select path"
    toColExp textValue = case parseJSONPath textValue of
      Left err     -> parseError $ T.pack $ "parse json path error: " ++ err
      Right []     -> pure Nothing
      Right jPaths -> pure $ Just $ IR.ColumnOp PG.jsonbPathOp $ PG.SEArray $ map elToColExp jPaths
    elToColExp (Key k)   = PG.SELit k
    elToColExp (Index i) = PG.SELit $ tshow i

orderByOperators
  :: NonEmpty (Definition P.EnumValueInfo, (BasicOrderType ('Postgres pgKind), NullsOrderType ('Postgres pgKind)))
orderByOperators = NE.fromList
  [ ( define $$(G.litName "asc") "in ascending order, nulls last"
    , (PG.OTAsc, PG.NLast)
    )
  , ( define $$(G.litName "asc_nulls_first") "in ascending order, nulls first"
    , (PG.OTAsc, PG.NFirst)
    )
  , ( define $$(G.litName "asc_nulls_last") "in ascending order, nulls last"
    , (PG.OTAsc, PG.NLast)
    )
  , ( define $$(G.litName "desc") "in descending order, nulls first"
    , (PG.OTDesc, PG.NFirst)
    )
  , ( define $$(G.litName "desc_nulls_first") "in descending order, nulls first"
    , (PG.OTDesc, PG.NFirst)
    )
  , ( define $$(G.litName "desc_nulls_last") "in descending order, nulls last"
    , (PG.OTDesc, PG.NLast)
    )
  ]
  where
    define name desc = P.mkDefinition name (Just desc) P.EnumValueInfo

comparisonExps
  :: forall pgKind m n r
   . ( BackendSchema ('Postgres pgKind)
     , MonadSchema n m
     , MonadError QErr m
     , MonadReader r m
     , Has QueryContext r
     )
  => ColumnType ('Postgres pgKind) -> m (Parser 'Input n [ComparisonExp ('Postgres pgKind)])
comparisonExps = P.memoize 'comparisonExps \columnType -> do
  -- see Note [Columns in comparison expression are never nullable]
  collapseIfNull <- asks $ qcDangerousBooleanCollapse . getter

  -- parsers used for comparison arguments
  geogInputParser    <- geographyWithinDistanceInput
  geomInputParser    <- geometryWithinDistanceInput
  ignInputParser     <- intersectsGeomNbandInput
  ingInputParser     <- intersectsNbandGeomInput
  typedParser        <- columnParser columnType (G.Nullability False)
  nullableTextParser <- columnParser (ColumnScalar PGText) (G.Nullability True)
  textParser         <- columnParser (ColumnScalar PGText) (G.Nullability False)
  -- `lquery` represents a regular-expression-like pattern for matching `ltree` values.
  lqueryParser       <- columnParser (ColumnScalar PGLquery) (G.Nullability False)
  -- `ltxtquery` represents a full-text-search-like pattern for matching `ltree` values.
  ltxtqueryParser    <- columnParser (ColumnScalar PGLtxtquery) (G.Nullability False)
  maybeCastParser    <- castExp columnType
  let name = P.getName typedParser <> $$(G.litName "_comparison_exp")
      desc = G.Description $ "Boolean expression to compare columns of type "
        <>  P.getName typedParser
        <<> ". All fields are combined with logical 'AND'."
      textListParser   = fmap openValueOrigin <$> P.list textParser
      columnListParser = fmap openValueOrigin <$> P.list typedParser

  pure $ P.object name (Just desc) $ fmap catMaybes $ sequenceA $ concat
    [ flip (maybe []) maybeCastParser $ \castParser ->
      [ P.fieldOptional $$(G.litName "_cast")    Nothing (ACast <$> castParser)
      ]

    -- Common ops for all types
    , equalityOperators
        collapseIfNull
        (mkParameter <$> typedParser)
        (mkListLiteral columnType <$> columnListParser)

    -- Comparison ops for non Raster types
    , guard (isScalarColumnWhere (/= PGRaster) columnType) *>
      comparisonOperators
        collapseIfNull
        (mkParameter <$> typedParser)

    -- Ops for Raster types
    , guard (isScalarColumnWhere (== PGRaster) columnType) *>
      [ mkBoolOperator collapseIfNull $$(G.litName "_st_intersects_rast")
        Nothing
        (ABackendSpecific . ASTIntersectsRast . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_intersects_nband_geom")
        Nothing
        (ABackendSpecific . ASTIntersectsNbandGeom <$> ingInputParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_intersects_geom_nband")
        Nothing
        (ABackendSpecific . ASTIntersectsGeomNband <$> ignInputParser)
      ]

    -- Ops for String like types
    , guard (isScalarColumnWhere isStringType columnType) *>
      [ mkBoolOperator collapseIfNull $$(G.litName "_like")
        (Just "does the column match the given pattern")
        (ALIKE     . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_nlike")
        (Just "does the column NOT match the given pattern")
        (ANLIKE    . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_ilike")
        (Just "does the column match the given case-insensitive pattern")
        (ABackendSpecific . AILIKE . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_nilike")
        (Just "does the column NOT match the given case-insensitive pattern")
        (ABackendSpecific . ANILIKE . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_similar")
        (Just "does the column match the given SQL regular expression")
        (ABackendSpecific . ASIMILAR  . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_nsimilar")
        (Just "does the column NOT match the given SQL regular expression")
        (ABackendSpecific . ANSIMILAR . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_regex")
        (Just "does the column match the given POSIX regular expression, case sensitive")
        (ABackendSpecific . AREGEX  . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_iregex")
        (Just "does the column match the given POSIX regular expression, case insensitive")
        (ABackendSpecific . AIREGEX . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_nregex")
        (Just "does the column NOT match the given POSIX regular expression, case sensitive")
        (ABackendSpecific . ANREGEX  . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_niregex")
        (Just "does the column NOT match the given POSIX regular expression, case insensitive")
        (ABackendSpecific . ANIREGEX . mkParameter <$> typedParser)
      ]

    -- Ops for JSONB type
    , guard (isScalarColumnWhere (== PGJSONB) columnType) *>
      [ mkBoolOperator collapseIfNull $$(G.litName "_contains")
        (Just "does the column contain the given json value at the top level")
        (ABackendSpecific . AContains    . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_contained_in")
        (Just "is the column contained in the given json value")
        (ABackendSpecific . AContainedIn . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_has_key")
        (Just "does the string exist as a top-level key in the column")
        (ABackendSpecific . AHasKey      . mkParameter <$> nullableTextParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_has_keys_any")
        (Just "do any of these strings exist as top-level keys in the column")
        (ABackendSpecific . AHasKeysAny . mkListLiteral (ColumnScalar PGText) <$> textListParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_has_keys_all")
        (Just "do all of these strings exist as top-level keys in the column")
        (ABackendSpecific . AHasKeysAll . mkListLiteral (ColumnScalar PGText) <$> textListParser)
      ]

    -- Ops for Geography type
    , guard (isScalarColumnWhere (== PGGeography) columnType) *>
      [ mkBoolOperator collapseIfNull $$(G.litName "_st_intersects")
        (Just "does the column spatially intersect the given geography value")
        (ABackendSpecific . ASTIntersects . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_d_within")
        (Just "is the column within a given distance from the given geography value")
        (ABackendSpecific . ASTDWithinGeog <$> geogInputParser)
      ]

    -- Ops for Geometry type
    , guard (isScalarColumnWhere (== PGGeometry) columnType) *>
      [ mkBoolOperator collapseIfNull $$(G.litName "_st_contains")
        (Just "does the column contain the given geometry value")
        (ABackendSpecific . ASTContains   . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_crosses")
        (Just "does the column cross the given geometry value")
        (ABackendSpecific . ASTCrosses    . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_equals")
        (Just "is the column equal to given geometry value (directionality is ignored)")
        (ABackendSpecific . ASTEquals     . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_overlaps")
        (Just "does the column 'spatially overlap' (intersect but not completely contain) the given geometry value")
        (ABackendSpecific . ASTOverlaps   . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_touches")
        (Just "does the column have atleast one point in common with the given geometry value")
        (ABackendSpecific . ASTTouches    . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_within")
        (Just "is the column contained in the given geometry value")
        (ABackendSpecific . ASTWithin     . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_intersects")
        (Just "does the column spatially intersect the given geometry value")
        (ABackendSpecific . ASTIntersects . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_3d_intersects")
        (Just "does the column spatially intersect the given geometry value in 3D")
        (ABackendSpecific . AST3DIntersects . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_d_within")
        (Just "is the column within a given distance from the given geometry value")
        (ABackendSpecific . ASTDWithinGeom <$> geomInputParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_st_3d_d_within")
        (Just "is the column within a given 3D distance from the given geometry value")
        (ABackendSpecific . AST3DDWithinGeom <$> geomInputParser)
      ]

    -- Ops for Ltree type
    , guard (isScalarColumnWhere (== PGLtree) columnType) *>
      [ mkBoolOperator collapseIfNull $$(G.litName "_ancestor")
        (Just "is the left argument an ancestor of right (or equal)?")
        (ABackendSpecific . AAncestor        . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_ancestor_any")
        (Just "does array contain an ancestor of `ltree`?")
        (ABackendSpecific . AAncestorAny     . mkListLiteral columnType <$> columnListParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_descendant")
        (Just "is the left argument a descendant of right (or equal)?")
        (ABackendSpecific . ADescendant      . mkParameter <$> typedParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_descendant_any")
        (Just "does array contain a descendant of `ltree`?")
        (ABackendSpecific . ADescendantAny   . mkListLiteral columnType <$> columnListParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_matches")
        (Just "does `ltree` match `lquery`?")
        (ABackendSpecific . AMatches         . mkParameter <$> lqueryParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_matches_any")
        (Just "does `ltree` match any `lquery` in array?")
        (ABackendSpecific . AMatchesAny      . mkListLiteral (ColumnScalar PGLquery) <$> textListParser)
      , mkBoolOperator collapseIfNull $$(G.litName "_matches_fulltext")
        (Just "does `ltree` match `ltxtquery`?")
        (ABackendSpecific . AMatchesFulltext . mkParameter <$> ltxtqueryParser)
      ]
    ]
  where
    mkListLiteral :: ColumnType ('Postgres pgKind) -> [ColumnValue ('Postgres pgKind)] -> UnpreparedValue ('Postgres pgKind)
    mkListLiteral columnType columnValues = P.UVLiteral $ SETyAnn
      (SEArray $ txtEncoder . cvValue <$> columnValues)
      (mkTypeAnn $ CollectableTypeArray $ unsafePGColumnToBackend columnType)

    castExp :: ColumnType ('Postgres pgKind) -> m (Maybe (Parser 'Input n (CastExp ('Postgres pgKind) (UnpreparedValue ('Postgres pgKind)))))
    castExp sourceType = do
      let maybeScalars = case sourceType of
            ColumnScalar PGGeography -> Just (PGGeography, PGGeometry)
            ColumnScalar PGGeometry  -> Just (PGGeometry, PGGeography)
            _                        -> Nothing

      forM maybeScalars $ \(sourceScalar, targetScalar) -> do
        sourceName   <- mkScalarTypeName sourceScalar <&> (<> $$(G.litName "_cast_exp"))
        targetName   <- mkScalarTypeName targetScalar
        targetOpExps <- comparisonExps $ ColumnScalar targetScalar
        let field = P.fieldOptional targetName Nothing $ (targetScalar, ) <$> targetOpExps
        pure $ P.object sourceName Nothing $ M.fromList . maybeToList <$> field

geographyWithinDistanceInput
  :: forall pgKind m n. (MonadSchema n m, MonadError QErr m)
  => m (Parser 'Input n (DWithinGeogOp (UnpreparedValue ('Postgres pgKind))))
geographyWithinDistanceInput = do
  geographyParser <- columnParser (ColumnScalar PGGeography) (G.Nullability False)
  -- FIXME
  -- It doesn't make sense for this value to be nullable; it only is for
  -- backwards compatibility; if an explicit Null value is given, it will be
  -- forwarded to the underlying SQL function, that in turns treat a null value
  -- as an error. We can fix this by rejecting explicit null values, by marking
  -- this field non-nullable in a future release.
  booleanParser   <- columnParser (ColumnScalar PGBoolean)   (G.Nullability True)
  floatParser     <- columnParser (ColumnScalar PGFloat)     (G.Nullability False)
  pure $ P.object $$(G.litName "st_d_within_geography_input") Nothing $
    DWithinGeogOp <$> (mkParameter <$> P.field $$(G.litName "distance") Nothing floatParser)
                  <*> (mkParameter <$> P.field $$(G.litName "from")     Nothing geographyParser)
                  <*> (mkParameter <$> P.fieldWithDefault $$(G.litName "use_spheroid") Nothing (G.VBoolean True) booleanParser)

geometryWithinDistanceInput
  :: forall pgKind m n. (MonadSchema n m, MonadError QErr m)
  => m (Parser 'Input n (DWithinGeomOp (UnpreparedValue ('Postgres pgKind))))
geometryWithinDistanceInput = do
  geometryParser <- columnParser (ColumnScalar PGGeometry) (G.Nullability False)
  floatParser    <- columnParser (ColumnScalar PGFloat)    (G.Nullability False)
  pure $ P.object $$(G.litName "st_d_within_input") Nothing $
    DWithinGeomOp <$> (mkParameter <$> P.field $$(G.litName "distance") Nothing floatParser)
                  <*> (mkParameter <$> P.field $$(G.litName "from")     Nothing geometryParser)

intersectsNbandGeomInput
  :: forall pgKind m n. (MonadSchema n m, MonadError QErr m)
  => m (Parser 'Input n (STIntersectsNbandGeommin (UnpreparedValue ('Postgres pgKind))))
intersectsNbandGeomInput = do
  geometryParser <- columnParser (ColumnScalar PGGeometry) (G.Nullability False)
  integerParser  <- columnParser (ColumnScalar PGInteger)  (G.Nullability False)
  pure $ P.object $$(G.litName "st_intersects_nband_geom_input") Nothing $
    STIntersectsNbandGeommin <$> (mkParameter <$> P.field $$(G.litName "nband")   Nothing integerParser)
                             <*> (mkParameter <$> P.field $$(G.litName "geommin") Nothing geometryParser)

intersectsGeomNbandInput
  :: forall pgKind m n. (MonadSchema n m, MonadError QErr m)
  => m (Parser 'Input n (STIntersectsGeomminNband (UnpreparedValue ('Postgres pgKind))))
intersectsGeomNbandInput = do
  geometryParser <- columnParser (ColumnScalar PGGeometry) (G.Nullability False)
  integerParser  <- columnParser (ColumnScalar PGInteger)  (G.Nullability False)
  pure $ P.object $$(G.litName "st_intersects_geom_nband_input") Nothing $ STIntersectsGeomminNband
    <$> (     mkParameter <$> P.field         $$(G.litName "geommin") Nothing geometryParser)
    <*> (fmap mkParameter <$> P.fieldOptional $$(G.litName "nband")   Nothing integerParser)

mkCountType :: Maybe Bool -> Maybe [Column ('Postgres pgKind)] -> CountType ('Postgres pgKind)
mkCountType _           Nothing     = PG.CTStar
mkCountType (Just True) (Just cols) = PG.CTDistinct cols
mkCountType _           (Just cols) = PG.CTSimple cols

-- | Various update operators
updateOperators
  :: forall pgKind m n r
   . (BackendSchema ('Postgres pgKind), MonadSchema n m, MonadTableInfo r m)
  => TableInfo ('Postgres pgKind)    -- ^ table info
  -> UpdPermInfo ('Postgres pgKind)  -- ^ update permissions of the table
  -> m (Maybe (InputFieldsParser n [(Column ('Postgres pgKind), IR.UpdOpExpG (UnpreparedValue ('Postgres pgKind)))]))
updateOperators tableInfo updatePermissions = do
  tableGQLName <- getTableGQLName tableInfo
  columns      <- tableUpdateColumns tableInfo updatePermissions
  let numericCols = onlyNumCols   columns
      jsonCols    = onlyJSONBCols columns
  parsers <- catMaybes <$> sequenceA
    [ updateOperator tableGQLName $$(G.litName "_set")
        typedParser IR.UpdSet columns
        "sets the columns of the filtered rows to the given values"
        (G.Description $ "input type for updating data in table " <>> tableName)

    , updateOperator tableGQLName $$(G.litName "_inc")
        typedParser IR.UpdInc numericCols
        "increments the numeric columns with given value of the filtered values"
        (G.Description $"input type for incrementing numeric columns in table " <>> tableName)

    , let desc = "prepend existing jsonb value of filtered columns with new jsonb value"
      in updateOperator tableGQLName $$(G.litName "_prepend")
         typedParser IR.UpdPrepend jsonCols desc desc

    , let desc = "append existing jsonb value of filtered columns with new jsonb value"
      in updateOperator tableGQLName $$(G.litName "_append")
         typedParser IR.UpdAppend jsonCols desc desc

    , let desc = "delete key/value pair or string element. key/value pairs are matched based on their key value"
      in updateOperator tableGQLName $$(G.litName "_delete_key")
         nullableTextParser IR.UpdDeleteKey jsonCols desc desc

    , let desc = "delete the array element with specified index (negative integers count from the end). "
                 <> "throws an error if top level container is not an array"
      in updateOperator tableGQLName $$(G.litName "_delete_elem")
         nonNullableIntParser IR.UpdDeleteElem jsonCols desc desc

    , let desc = "delete the field or element with specified path (for JSON arrays, negative integers count from the end)"
      in updateOperator tableGQLName $$(G.litName "_delete_at_path")
         (fmap P.list . nonNullableTextParser) IR.UpdDeleteAtPath jsonCols desc desc
    ]
  whenMaybe (not $ null parsers) do
    let allowedOperators = fst <$> parsers
    pure $ fmap catMaybes (sequenceA $ snd <$> parsers)
      `P.bindFields` \opExps -> do
        -- there needs to be at least one operator in the update, even if it is empty
        let presetColumns = Map.toList $ IR.UpdSet . partialSQLExpToUnpreparedValue <$> upiSet updatePermissions
        when (null opExps && null presetColumns) $ parseError $
          "at least any one of " <> commaSeparated allowedOperators <> " is expected"

        -- no column should appear twice
        let flattenedExps = concat opExps
            erroneousExps = OMap.filter ((>1) . length) $ OMap.groupTuples flattenedExps
        unless (OMap.null erroneousExps) $ parseError $
          "column found in multiple operators; " <>
          T.intercalate ". " [ dquote columnName <> " in " <> commaSeparated (IR.updateOperatorText <$> ops)
                             | (columnName, ops) <- OMap.toList erroneousExps
                             ]

        pure $ presetColumns <> flattenedExps
  where
    tableName = tableInfoName tableInfo
    typedParser columnInfo  = fmap P.mkParameter <$> columnParser (pgiType columnInfo) (G.Nullability $ pgiIsNullable columnInfo)
    nonNullableTextParser _ = fmap P.mkParameter <$> columnParser (ColumnScalar PGText)    (G.Nullability False)
    nullableTextParser    _ = fmap P.mkParameter <$> columnParser (ColumnScalar PGText)    (G.Nullability True)
    nonNullableIntParser  _ = fmap P.mkParameter <$> columnParser (ColumnScalar PGInteger) (G.Nullability False)

    onlyJSONBCols = filter (isScalarColumnWhere (== PGJSONB) . pgiType)

    updateOperator
      :: G.Name
      -> G.Name
      -> (ColumnInfo b -> m (Parser 'Both n a))
      -> (a -> IR.UpdOpExpG (UnpreparedValue b))
      -> [ColumnInfo b]
      -> G.Description
      -> G.Description
      -> m (Maybe (Text, InputFieldsParser n (Maybe [(Column b, IR.UpdOpExpG (UnpreparedValue b))])))
    updateOperator tableGQLName opName mkParser updOpExp columns opDesc objDesc =
      whenMaybe (not $ null columns) do
        fields <- for columns \columnInfo -> do
          let fieldName = pgiName columnInfo
              fieldDesc = pgiDescription columnInfo
          fieldParser <- mkParser columnInfo
          pure $ P.fieldOptional fieldName fieldDesc fieldParser
            `mapField` \value -> (pgiColumn columnInfo, updOpExp value)
        let objName = tableGQLName <> opName <> $$(G.litName "_input")
        pure $ (G.unName opName,)
             $ P.fieldOptional opName (Just opDesc)
             $ P.object objName (Just objDesc)
             $ catMaybes <$> sequenceA fields